Thermoplastic polyester resin (hereinafter called PET) is engineering plastics excellent in heat resistance, chemical resistance and weather resistance, and also superior in tensile strength and other mechanical properties. This resin is mass produced by blow forming or the like and used widely as liquid containers. The PET is a crystalline resin, so that it is difficult to make resin foam, but it is extruded and foamed by adding a crosslinking agent as disclosed in the U.S. Pat. No. 5,000,991 to obtain a foamed sheet, and this foamed sheet is heated and formed into a container, and is recently used as a heat resistant container usable in heating and cooking in a microwave oven.
Such containers find no applications after the contained merchandise is consumed. The system for recycling a huge quantity of used containers has been regulated by law in various nations in relation to environmental problems. In this background there is a mounting need for effectively reusing the recycled containers.
Moreover, the PET has a moisture absorbing property, and when the resin containing moisture is put in an extruder heated to high temperature, it is hydrolyzed and deteriorates, and therefore it must be dried to 300 ppm or less. For example, in the specification of WO95/15257, the trimming loss of PET foamed sheet is ground and reused. This ground material is dried preliminarily. In this drying process, usually, the ground material is put in a dryer and is heated for some time while circulating dry air. The ground material of foam is bulky and a large dryer is needed, and the heating time must be kept for at least several hours, and hence this drying process is complicated, and omission of this drying has been expected. Besides, the dried state varies significantly, and it leads to fluctuations of the supply materials.
As disclosed in Japanese Patent Publication (TOKKYO KOKOKU) No. 60-54850, it has been known to fuse polystyrene resin, evacuate and suck from the barrel of an extruder to remove volatile components such as residual monomer, and inject foaming agent, thereby extruding and foaming. Incidentally, it was unexpected that PET foam of high quality free from deterioration of PET can be continuously extruded and foamed by applying this method in recycled PET, without drying process of PET, as the moisture is sufficiently removed by evacuation and suction in the extruder.